JPH02182825A - Manufacture of round billet for manufacturing seamless martensitic stainless steel pipe - Google Patents

Abstract

PURPOSE:To obtain a round billet capable of perfectly avoiding the occurrence of internal surface defects at the time of piercing-rolling by forming the structure in the central part of an ingot of the stainless steel with a specific composition into a rolled structure by means of primary rolling and then reducing the amount of precipitation of delta-ferrite by means of soaking and holding at specific temp. CONSTITUTION:An ingot of a martensitic stainless steel having a composition containing, by weight, 0.12-0.30% C and 11-14% Cr is heated and subjected to primary rolling by means of ordinary blooming, etc., by which the cast structure in the central part is changed into rolled structure. The primary rolled stock is soaked and held again in a soaking furnace at a temp. of the delta-ferrite precipitation temp. on the equilibrium diagram of this steel or below and then finished into a round billet of the prescribed outside diameter by means of ordinary billeting. By using the above round billet as stock and carrying out piercing rolling on an inclined rolling system represented by Mannesmann piercer, the rate of the occurrence of internal surface defects at the time of piercing rolling can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing a round billet for producing seamless martensitic stainless steel pipes used for oil country tubular goods, line pipes, etc.

<Prior art> Martensitic seamless stainless steel pipes represented by 5US410 steel or 5US420 steel are CO! It is used for oil country tubular goods, geothermal country tubular goods, line pipes, etc. because it shows excellent corrosion resistance in corrosive environments including

When manufacturing seamless stainless steel pipes, Mannesmann processes a solid round billet, which is produced by heating a slab or plume cast in a mold or continuous casting machine, and finishing it to a specified diameter by blooming and groove rolling. It is generally pierced and rolled using the skew rolling method (as typified by Piasa).

However, some types of martensitic stainless steel have poor hot workability, and as a result, rolling defects such as cracks and flaking occur on the inner surface during the piercing rolling process in Mannesmann piercers, which causes severe deformation of the material. This caused significant problems in yield and productivity.

Rolling defects such as cracks and flaking on the inner surface that occur during punch-rolling of martensitic stainless steel with a Mannesmann piercer are caused by the fact that δ ferrite present in the round billet during punch-rolling reduces hot workability. It is known that this occurs due to

For the purpose of preventing the occurrence of defects on the inner surface during piercing rolling, round billets before piercing rolling are heated in a temperature range of 950°C to 1150°C, as disclosed in, for example, Japanese Patent Application Laid-Open No. 134630/1983. A method has been proposed in which the δ ferrite in the round billet is disappeared by soaking and holding for 30 minutes or more, preferably in a temperature range of 1000°C to 1100°C for 1 hour or more.

<Problems to be Solved by the Invention> However, in the above-described method of soaking the round billet at high temperature for a long time before piercing and rolling, the time in the heating furnace increases, productivity decreases, and the scale is reduced. Not only does the loss and fuel consumption rate increase, but the decarburization of the outer surface of the round billet progresses, and the precipitation of δ ferrite on the outer surface is promoted, reducing hot workability and causing damage to the outer surface. The present invention has been made in view of the above-mentioned circumstances, and is intended to reduce defects on the inner surface during Mannesmann Piaser piercing rolling. The purpose of the present invention is to provide a method for manufacturing martensitic stainless steel round billets that can be manufactured with high productivity, low cost, and high yield while completely avoiding the occurrence of such occurrence.

<Means for Solving the Problems> As a result of intensive investigation and research into the relationship between the amount of δ ferrite remaining in a round billet after billet rolling of martensitic stainless steel and billet rolling conditions, the present inventors have discovered the following: After the slab or bloom, which is the raw material of the round billet, is subjected to primary rolling such as blooming to give the center a rolled structure, it is again soaked at a temperature below the precipitation temperature of δ ferrite on the equilibrium diagram, and then subjected to normal billet rolling. It was discovered that δ ferrite could be effectively eliminated by finishing it into a round billet, and based on this knowledge, the present invention was accomplished.

That is, in the present invention, C: 0.12 to 0.30%,
After heating a slab of martensitic stainless steel containing Cr: 11 to 14% by weight, the cast fibers in the center are changed to a rolled structure through primary rolling such as ordinary blooming rolling, and ■ this primary The rolled material is again soaked in a soaking furnace at a temperature below the δ-ferrite precipitation temperature on the equilibrium diagram for this steel, and then finish rolled into a round billet with a predetermined outer diameter using normal billet rolling. This achieves the above objective.

Function> The present invention will be explained in detail below. Martensitic stainless steel having the composition shown in Table 1 manufactured by the normal converter continuous casting manufacturing process was heated under the conditions shown in Table 2. Round billet rolling was carried out.

As a result, it was found that the amount of delta ferrite precipitation was higher in the round billet obtained by billet-rolling the primary rolled material after the primary rolling with a rolling reduction ratio of 1.3 or more in a soaking furnace again at a temperature below the delta ferrite precipitation temperature. It was found that there was a significant decrease.

Here, in the billet manufacturing process described above, the reason why the amount of δ ferrite precipitation in the round billet after billet rolling is significantly reduced by soaking and holding the second rolling material is considered to be due to the following reason.

When the slab, which is the raw material for round billets, solidifies, the primary crystal is δ ferrite, which transforms into austenite during the subsequent cooling process, but due to the segregation of ferrite former elements such as Cr+P that exist near the center of the slab. , δ ferrite in the segregated area remains without being transformed into austenite. When producing billets using slabs with residual δ ferrite as raw material, even if the slab is heated at a temperature at which δ ferrite does not precipitate on the equilibrium diagram (see Figure 2), Cr+P, etc. Segregation does not easily diffuse, and δ ferrite does not disappear, but remains in the round billet as it is.

However, when the primary rolled material whose center part had changed from the cast structure to the rolled structure due to the second rolling was heated and held again at a temperature at which δ ferrite does not precipitate on the equilibrium phase diagram, the strain was added by the second rolling. The rolled structure starts to recrystallize, and this recrystallization promotes the diffusion of segregation near the center, and δ
Ferrite easily transforms into austenite, reducing the amount of δ ferrite remaining in the round billet.

Here, in order for the central part of the secondary rolled material to have a rolled structure and be easily recrystallized by subsequent reheating, the rolling reduction ratio when blooming from the raw slab to the primary rolled material is 1.
3 or more is required, and preferably 2° or more is effective. However, if the rolling reduction ratio is too large, the load during rolling will increase, the number of passes will increase, the temperature of the material will drop, the workability will deteriorate, and surface defects will occur frequently in the next rolling, so the reduction will increase. A ratio of 10 or less is realistic.

Furthermore, in order to promote the diffusion of segregation and the transformation of δ ferrite into austenite in the -th rolled material, it is necessary to reheat the material just below the boundary temperature between the austenite single-phase region and the ferrite-austenite dual-phase region on the equilibrium phase diagram of the material.・It is necessary to maintain uniform heating, but considering segregation of materials, variations in furnace temperature, etc., it is preferable to maintain uniform heating at a temperature 30 to 200°C lower than the above boundary temperature, and the holding time is 30 minutes or more, preferably 2 hours or more, but 1
If it exceeds 0 hours, the effect will be saturated, and not only will the scale loss amount and fuel consumption rate increase, but also the precipitation of δ ferrite due to surface decarburization will be promoted, and surface rolling defects will occur during billet mill rolling after reheating. Therefore, it is desirable to set the upper limit to 10 hours. Furthermore, if the holding time is 10 hours or less, the depth of decarburization on the surface of the round billet after rolling will not interfere with piercing rolling with a Mannesmann piercer. It can be kept to a certain extent.

<Example> Examples of the present invention will be described below.

Table 3 Steel type A of 13% Cr steel having the composition shown in Table 3,
Three types, B and C, were manufactured into slab materials with the respective cross sections shown in Table 4 using a conventional converter continuous casting method. The slabs were finished into billets of 207 waφ according to the manufacturing process shown in Figure 1 and under the conditions shown in Table 4, and then milled into billets of 139.7 vmφ using the Mannesmann-mandrel mill method.
Manufactured into a seamless pipe of X7.72mm.

This tube was subjected to visual inspection and ultrasonic flaw detection to investigate the incidence of internal defects. The results are also shown in Table 4.

As is clear from this table, the incidence of internal defects in tubes using round billets manufactured by the method of the present invention is significantly reduced compared to tubes using round billets manufactured by the conventional method. I know that there is.

<Effects of the Invention> As explained above, according to the present invention, C: 0°12~
Using a slab of martensitic stainless steel containing 0.30% by weight and 11 to 14% by weight of Cr as a raw material, the slab was normally rolled to a reduction ratio of 1.3 or more, and then After soaking again at a temperature below the boundary temperature between the austenite single-phase region and the ferrite-austenite dual-phase region, a round billet for seamless steel pipe production is produced by normal rolling. It is possible to reduce the incidence of defects, reduce maintenance man-hours, improve productivity, and reduce costs, making it possible to manufacture high-quality seamless steel pipes at low cost.

[Brief explanation of the drawing]

Figure 1 is a conceptual diagram schematically showing the manufacturing process of the present invention for rolling a round billet, which is the raw material for seamless martensitic stainless steel pipes, and Figure 2 is a diagram showing the carbon content and temperature of 13% Cr steel. FIG. 1...Converter. 2... Continuous casting machine. 3... Ingot. 4...Blooding mill. 5... Slab. 6... Heating furnace. 7...Blooding mill. 8... Reheating furnace. 9... Billet rolling machine. 10...Round billet. Patent applicant: Kawasaki Steel Corporation

Claims (1)

[Claims] When manufacturing a round billet for seamless steel pipe production using a martensitic stainless steel slab containing 0.12 to 0.30% by weight of C and 11 to 14% by weight of Cr, the slab is rolled down. A material normally rolled at a ratio of 1.3 or more is reheated and kept at a temperature below the boundary temperature between the austenite single phase region and the ferrite-austenite two phase region, and then rolled into a round billet by normal rolling. A method for manufacturing a round billet for manufacturing seamless martensitic stainless steel pipes, characterized by: